This application is an improvement of U.S. patent application Ser. No. 14/326,969 filed Jul. 9, 2014 entitled “METHOD TO EXTEND THE NUMBER OF CONSTANT BITS EMBEDDED IN AN INSTRUCTION SET.”
Constants are commonly used in calculations. Such constants must be provided by the program to the instruction making computations. Such constants may be provided by storing them in memory, loading them into a register of a central processing unit register set and performing the needed computation using the register stored constant in conjunction with register stored variables.
As an alternative for little used constants, they can be embedded in instructions. The typical manner of specifying such constants is replacing an input operand field with the constant bits. Such an input operand field typically includes a number of a register storing the corresponding input operand. An immediate instruction treats this input operand field as a constant. This is called an immediate field. This severely limits the size of such constants due to the limited number of bits available. A typical central processing unit will have 16 or 32 general purpose registers. This limits the constant that can be specified by an immediate field to 4 or 5 bits.
A technique used in the prior art to specify constants employs variable length instruction words. Using variable length instruction words, the length of an instruction can be extended to provide a needed number of bits for specifying a constant. This is sub optimal because a fixed word length instruction set architecture requires fewer hardware resources to decode.
U.S. patent application Ser. No. 14/326,969 filed Jul. 9, 2014 entitled “METHOD TO EXTEND THE NUMBER OF CONSTANT BITS EMBEDDED IN AN INSTRUCTION SET” retained a fixed length instruction set. This patent provided special instruction for constant extension bits. The instruction included constant extension bits in predetermined bit locations. If an instruction included an indication of constant extension, the decoder employed bits at a predetermined bit position in an adjacent instruction to extend the constant of an immediate field. The patent application taught concatenating bits from these two sources to produce the longer constant.